Answers

Is the device staying powered on? Do you hear any lock sounds or notice any activity when the screen goes black? Does the device boot loop or just turn black? If the device is boot looping you may have damaged the gas gauge components that are next to the battery connector when using your tool to disconnect it. Let me know what you find!

I've seen plenty of cheap aftermarket screens just randomly quit. Test with a knowingly good LCD assembly and we will go from there. Sometimes problems are the most obvious and sometimes they aren't. Lets take it from the top! Hope this helps!!

Take a look at the screen very closely, in some cases if the LCD connector is removed while the mainboard is connected to the battery you can damage the backlight filter. In this case I would start by cleaning the screen very well and looking at it under good light (reduce as much glare as possible) and see if you can spot the apple logo after a soft/hard reset. If you can see the faint apple logo you would either need to test the backlight circuitry or send it off to be repaired. I personally micro solder and have seen this case many times. most of the time a backlight filter is replaced and the device lights right up! Check for a logo and get back to me and we will work from there! Hope this helps!!!!

This is a common issue, the adhesive between the glass and plastic frame has come loose and separating. A screen replacement will be a straight forward solution, but if you'd like a "quick fix" just get a small amount of super glue on a toothpick and apply a bit between the glass and plastic frame. Make sure not to adhere the screen assembly frame to the metal housing of the phone.

Was there a repair done on the device? Sounds like you may have a (Gas gauge) issue with the battery. If the phone cannot receive gas gauge info it will boot loop(iPhone 5 and up need battery connected to boot)!

Looks like you've detached the ground terminals that complete your SWI(Gas Guage) circuit - During boot the device searches for battery information, if it gets no info it will boot loop - The connector will need to be replaced - Sure, you could rig something up but I don't recommend it!

Your flex cable seems to be defective - There are 2 gold pads on that flex that your speaker sits on (assuming your referring to your loud speaker) make sure those are in contact with the pins - If they are in contact you either slightly tore copper traces in the cable or its a defect - Done repairs for years and ive seen this multiple times! Hope this helps!

Resources

Repairability

Stay in the loop

It's time to speak out for your right to repair

We have a chance to guarantee our right to repair electronic
equipment—like smartphones, computers, and even farm equipment. This is a once-in-a-generation
chance to protect local repair jobs—the corner mom-and-pop repair shops that
keep getting squeezed out by manufacturers.

Join the cause and tell your state representative to support Right to Repair. Tell them you believe repair should be fair, affordable, and accessible. Stand up for your right to repair!